Such a noise reduction arrangement is known from J. Guo, e.a, “Actively created quiet zones by multiple control sources in free space”, J. Acoust. Soc. Am. 101 (3), March 1997, pp. 1492–1501. This document discloses an arrangement with a series of secondary sources on a first line and a series of error sensors on a second line, the first and second lines being parallel. The primary concern of this document is to create large areas of quiet zones. The document observes that such a requirement can be satisfied if the error sensors are not in the near field of the secondary sources. According to the document, the distance between the second line with the error sensors and the first line with the secondary sources should be greater than or comparable to the mutual distances between the secondary sources. Guo e.a. only present a model for this two line arrangement. Moreover, in their model, all secondary sources are controlled by the output signals of all error sensors. Implementing such a control arrangement results in a complex controller with many connections and which turns out to be rather slow in many applications.
S. J. Elliott et al., Interaction Between Multiple Feedforward Active Control Systems, IEEE Transactions on Speech and Audio Processing, Vol. 2, No. 4, 1994, pp. 521–530 [1] describe a noise reduction system having a panel of actuators arranged in a first plane and a plurality of error sensors in a second plane. The first and second planes are parallel to one another. Elliott et al. present a mathematical model of a decentralised adaptive feedforward control system. They also present results of some physical examples in which there are two actuators and two error sensors. In these examples, Elliott et al. introduce the mutual distances between the error sensors and the actuators as important parameters to derive conditions as to when such a system is stable. In the physical examples given, the distance between the two planes is about 0.3 times the distance between the two actuators. Elliott et al. do not disclose the presence of an optimum distance between the two planes as a function of the mutual distance between actuators.
X. Qui, e.a, A Comparison of Near-field Acoustic Error Sensing Strategies for the Active Control of Harmonic Free Field Sound Radiation, Journal of Sound and Vibration, 1998, 215(1), pp. 81–103 [2], disclose the results of a study to find the best location of an error sensor relative to a primary noise source. However, this study is limited to a harmonic sound field radiated by a monopole primary source and by a dipole-like pair of primary sources. In both cases the actuator is a monopole radiating at the same frequency as the primary source. No plurality of actuators and plurality of error sensors arranged in respective planes are disclosed.
An active high transmission loss panel is disclosed in WO-A-94/05005. However, in this patent document the actuators and sensors are all located in the same plane.